First, if they don't increase HP, the fight will be too easy. Games should primarily be fun. If players miss too often, it can be frustrating for them. It problem with turn-based games which you can't easly fix by adding extra enemies because fights will be to long.
Another thing that is not fun is when half of your spell list is useless because enemies HP is too high, or AC is so low that chances to hit it with a mace are higher.
